I can use it when I speak and write, but except for certain situations where it's always used (like after "para que" and "cuando" when you're talking about the future and sentences beginning with "quiero que ...."), I often have to stop and think to myself, "Do I need the subjunctive here?"

So, I know there is such a thing as the subjunctive in Spanish, I know that it often occurs after certain negated verbs, so I try out the sentence “No quiero que el lo haga” and see how it is received.